The Doors of Oerth
Acererak the Devourer was born a cambion, the product of a balor named Tarnhem who took his human mother by force. Apparently an ancient conjurer summoned a demon that was far beyond his ability to control. The demon devoured the wizard, and forcibly conceived a cambion son in one of the wizard’s servant girls.
Acererak’s mother survived her son’s birth, but she was killed by a torch-wielding mob ten years later. The boy was rescued by none other than Vecna, the Whispered One, who killed the advisors that urged him to kill the child and instead took the half-demon on as an apprentice. Even then, ten year-old Acererak loathed life, looking forward to becoming undead like his master. During Vecna’s siege of Fleeth, the lich was severely wounded and rescued by Acererak. Vecna subsequently promoted the cambion.
It is unknown if Acererak was present when Vecna was betrayed by Kas the Bloody-Handed in the fourth century before the Common Year, but at some point, he moved to the Vast Swamp where he constructed a lair for himself, colloquially known as the Tomb of Horrors (and described in an adventure module of the same name). While alive, Acererak built a subterranean temple complex in the name of Orcus, burying its architect and all of its workers within. Eventually, Acererak succumbed to the lure of lichcraft, and had himself buried in a labyrinthine tomb where he committed himself to his studies and, eventually, demilichdom, abandoning his body for the planes beyond. Many adventurers over the years have attempted to raid Acererak’s tomb, but it is believed that none have been able to destroy him, despite some claims.